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- [Voiceover] To change the speed of sound

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you have to change the
properties of the medium

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that sound wave is traveling through.

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There's two main factors about a medium

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that will determine the
speed of the sound wave

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through that medium.

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One is the stiffness of the
medium or how rigid it is.

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The stiffer the medium
the faster the sound waves

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will travel through it.

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This is because in a stiff material,

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each molecule is more interconnected

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to the other molecules around it.

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So any disturbance gets
transmitted faster down the line.

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The other factor that determines
the speed of a sound wave

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is the density of the medium.

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The more dense the medium,

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the slower the sound wave
will travel through it.

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This makes sense because if
a material is more massive

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it has more inertia and
therefore it's more sluggish

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to changes in movement or oscillations.

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These two factors are taken
into account with this formula.

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V is the speed of sound.

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Capital B is called the bulk
modulus of the material.

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The bulk modulus is the
official way physicists measure

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how stiff a material is.

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The bulk modulus has units of pascals

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because it's measuring how
much pressure is required

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to compress the material
by a certain amount.

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Stiff, rigid materials
like metal would have

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a large bulk modulus.

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More compressible
materials like marshmallows

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would have a smaller bulk modulus.

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Row is the density of the material

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since density is the mass per volume,

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the density gives you
an idea of how massive

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a certain portion of
the material would be.

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For example, let's
consider a metal like iron.

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Iron is definitely more
rigid and stiff than air

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so it has a much larger
bulk modulus than air.

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This would tend to make
sound waves travel faster

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through iron than it does through air.

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But iron also has a much
higher density than air,

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which would tend to make sound waves

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travel slower through it.

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So which is it?

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Does sound travel faster
though iron or slower?

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Well it turns out that the
higher stiffness of iron

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more than compensates
for the increased density

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and the speed of sound through iron

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is about 14 times faster than through air.

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This means that if you were to place

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one ear on a railroad
track and someone far away

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struck the same railroad
track with a hammer,

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you should hear the noise 14 times faster

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in the ear placed on the track compared

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to the ear just listening through the air.

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In fact, the larger bulk
modulus of more rigid materials

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usually compensates for
any larger densities.

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Because of this fact, the
speed of sound is almost always

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faster through solids
than it is through liquids

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and faster through liquids
than it is through gases

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because solids are more rigid than liquids

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and liquids are more rigid than gases.

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Density is important in
some aspects too though.

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For instance, if you heat
up the air that a sound wave

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is travelling through, the
density of the air decreases.

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This explains why sound travels
faster through hotter air

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compared to colder air.

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The speed of sound at 20 degrees Celsius

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is about 343 meters per second,

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but the speed of sound
at zero degrees Celsius

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is only about 331 meters per second.

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Remember, the only way to
change the speed of sound

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is to change the properties of
the medium it's travelling in

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and the speed of sound is
typically faster through solids

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than it is through liquids

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and faster through liquids
than it is through gases.